A receiver that needs to demodulate a desired signal in a communication system must always deal with extracting the desired signal from noise. At one time, so-called “white noise” presented the greatest concern. White noise is characterized in that the noise “signal” is random, or uncorrelated, in time and space, meaning that the amplitude, phase, and other measurable characteristics of the noise at any one time or place cannot be used to predict the characteristics at any other time and place. Methods of designing a receiver to deal with white noise are well-known and have been in existence for decades.
Modern communication systems, such as wireless time domain multiplexed access (TDMA) systems increasingly have to deal with so-called “colored noise” which is correlated and has predictable characteristics. Co-channel interference is one example of colored noise. Co-channel interference has become a significant problem as the population density of people using wireless, cellular communication devices increases. Because of the traffic demand, it is necessary to re-use channels more frequently on a geographic basis. Co-channel interference can cause problems in different sections of a TDMA receiver. Such interference can cause difficulty with the timing estimation used for synchronization in the initial stage of processing a received signal. It can also create errors filtering and demodulating the signal in subsequent stages.
FIG. 1 illustrates the received signal at wireless terminal 101 where there is both co-channel interference and white noise present. Base station 102 is transmitting a desired signal over a channel designated Cd and base station 103 is transmitting an interfering signal or “interferer” over a channel designated Ci. Other interference is simply referred to as “noise” n. Given that each channel has specific characteristics that are represented with complex valued quantities, the total received signal at wireless terminal 101 can be represented as:r=Cd{circle around (x)}Sd+Ci{circle around (X)}Si+n,where Sd is the desired signal and Si is the interferer. Note that r is complex valued even where the signals, Sd and Si are real-valued. FIG. 2 illustrates the spectrums of the desired signal and the interferer, 201 and 202 respectively, in the frequency domain.
One known way to deal with co-channel and other colored interference in communication systems is to use some type of whitening filter to “whiten” the interference, so that the desired signal now only needs to be distinguished from white noise. Temporal whitening involves removing temporal correlation from the interfering signal and requires knowledge of the interfering signal's characteristics at different points in time. Spatial whitening involves removing spatial correlation from the interfering signal and requires knowledge of the interfering signal's characteristics at different points in space. True, spatial whitening in wireless systems requires multiple antennas. A signal is said to have a different “diversity branch” for each antenna. The two types of whitening can be combined, resulting in whitening in both time and space, commonly called “spatio-temporal” whitening. With any kind of whitening, knowledge of the characteristics of the channel is also needed to extract and demodulate the desired signal from the whitened signal.
In TDMA systems, such as those that implement the well-known Global System for Mobile (GSM), General Packet Radio Service (GPRS), and Enhanced General Packet Radio Service (EGPRS) standards and their various incarnations, a signal is received as a stream of timeslots. Each timeslot contains a known training sequence, also called a “sync word.” FIG. 3 illustrates such a timeslot, with training sequence, 301. Since the contents of the training sequence is known, the characteristics of the desired and interfering signals can be isolated in time and space, and used for spatio-temporal whitening just prior to demodulating the desired signal, using multiple antennas to perform the spatio-temporal whitening. Co-channel interference is also a problem for mobile terminals. However, since only one antenna is available, spatial or spatio-temporal whitening is difficult. Additionally, spatio-temporal whitening just prior to demodulating does not increase synchronization accuracy in the presence of co-channel interference.